It is a common practice for chemicals such as those used for cleaning and sanitizing to be purchased as concentrated liquids. The chemicals are mixed with water to achieve the desired usage concentration. A variety of proportioning dispensers have been developed to achieve this. These dispense mixtures at use concentration. The dispensers often employ venturi devices sometimes called eductors to proportion the chemical and deliver this for use. Water traveling through the central portion of the venturi creates suction which draws the chemical into the water stream. The amount of chemical educted is controlled by a metering orifice in the chemical feed line.
The concentrations desired in this type of chemical dispensing varies greatly ranging from 1:1 to over 1:1000. The devices also must function with a wide range of water pressures, temperatures and dissolved minerals and gases. In some of these conditions, the eductors function much like classical flow venturies, while in other they are more like jet pumps. The devices are mechanically simple, generally without moving parts, but small details of the construction have important influence on their performance.
It is usually desirable to operate these dispensers with water provided directly from the public water supply. In this situation, the dispensers are subject to the regulations of the public water departments who are concerned about preventing any possibility of the chemical concentrates entering the water system. Such an event is known as back flow when caused by positive pressure such as from a boiler. It is referred to as back syphoning when the flow is caused by suction in the water system.
A variety of devices and techniques exist to prevent backflow and back syphoning. The most effective mechanical backflow devices and the ones most accepted by the public water departments are relatively large, expensive devices which require regular testing and certification. The installation and inspection of these devices is often more expensive than the acquisition and installation of the dispensers themselves.
The regulations regarding backflow and back syphoning and the research supporting them generally recognize the simple air gap is the most effective protection of all. The simplest illustration of an air gap is a faucet whose end is above the top of the sink. If there is any suction from the water system, it cannot pull in anything from the sink, only air.
It is known to combine a venturi eductor with an air gap for back syphoning protection for dispensing applications. Such devices are described in U.S. Pat. Nos. 4,697,610 and 3,166,086 as well as U.S. Pat. Nos. 3,072,137 and 3,273,866. These function in specific applications. However, their air gaps are generally less than half an inch, and many standards require that the air gap be at least one inch.
In such applications where such a large air gap is employed, it is difficult to control the proportioning of the venturi and also difficult to prevent collateral spray from being emitted from the air gap.
Devices that include baffling to prevent collateral spray are disclosed in Kunstorff U.S. Pat. No. 2,288,247 and Boosey U.S. Pat. No. 2,250,291. Neither of these devices are directed at chemical eductors and therefore they have no concern with effectively proportioning the educted chemical. Further, the structures disclosed in these devices would be unsuitable for chemical eductors. The geometry for a chemical eductor is very precise.
The essential geometry of a venturi is that of an enlargement in a contained stream of fluid. According to Bernoulli's theory, suction is created at the point where the flow channel widens. The operation of the venturi requires that the entering fluid stream have a certain amount of flow energy. For an air gap eductor, this means that the stream must cross the air gap and enter the venturi developing appreciable pressure within the entrance of the venturi.
The geometry which will create this includes a nozzle diameter somewhat larger than the smallest diameter of the front part of the venturi along with a funnel structure leading to this venturi orifice. Not all the water volume from a water jet can enter the venturi and some degree of overflow is created.
The performance of the nozzle is critical for the correct operation of the unit. It must discharge a well defined stream across the air gap and into the inductor.
Such concerns are not present in siphon breakers and back flow preventors for water systems which are merely concerned with backflow. Such critical dimensions are certainly not a problem for chemical eductors that have relatively small air gaps or where those where overspray is not a critical concern.